1. Field of the Invention
The present invention relates to a concealment signal generator, a concealment signal generation method, and a computer product that generate concealment signals for missing voice-transmission-signals, and more particularly, to a concealment signal generator, a concealment signal generation method, and a computer product that can generate signals with minimal sound quality deterioration.
2. Description of the Related Art
Conventionally, in a voice signal transmission by voice over Internet protocol (VoIP), when there are missing voice-transmission-signals due to a cause such as a transmission error, a method is used by which the missing voice-transmission-signals are concealed by generating substitute signals that replace the missing voice-transmission-signals, thus preventing interrupted voice (see Japanese Patent Application Laid-open No. 2004-138756, Japanese translation of PCT international application (kohyo) No. 2002-542521, and Japanese Patent Application Laid-open No. 2005-338200). Such substitute signals are called concealment signals.
A wave replication (WR) method and a pitch wave replication (PWR) method are known methods for generating the concealment signals. The WR method uses properly transmitted voice-transmission-signals, and generates the concealment signals by repeating a sound waveform at a position where a correlation with a waveform preceding the lost signal is large. PWR uses properly transmitted voice-transmission-signals, and generates the concealment signals by repeating a pitch waveform of one cycle preceding the loss.
However, when the concealment signals generated by the aforementioned conventional methods are used, an abnormal buzz-like noise is generated as a result of the repetition of the same waveform.
FIG. 15 is a schematic for explaining the problem related to the conventional concealment signal generation method and shows a concealment signal waveform when PWR method is used. As shown in FIG. 15, a last pitch waveform 3 of a section where the frame is transmitted properly (normal section) is repeated in a section where there are lost frames with no voice-transmission-signals (lost-frame section). Consequently, an unnatural buzz-like sound is heard due to the repetition of transmission of waveform of the same pitch and continuation of an unvarying sound.